1. Field of the Invention
The present invention relates to a method for forming a silicon oxide film. More specifically, the present invention relates to a method for forming a silicon oxide film having improved quality by using a radical shower CVD system (RS-CVD system).
2. Description of the Related Art
At present, plasma CVD systems are employed to form silicon oxide suitable to a gate insulating film at low temperature in manufacturing a liquid crystal display which employs a low temperature polycrystalline silicon TFT.
Among those plasma CVD systems, there is known a CVD system proposed in Japanese Patent Unexamined Application Publication No. 2000-345349(JP-A-2000-345349), which is a prior patent application to this application. In this specification, this CVD system is referred to as “RS-CVD system”, which is a Radical Shower CVD system, so as to differentiate this CVD system proposed in JP-A-2000-345349 from the ordinary CVD system. This RS-CVD system generates plasma in a vacuum container to generate electrically neutral, excited, active species (REFERRING to these electrically neutral, excited active species as “radicals” herein after in this specification) and form a film on a substrate by the radicals and material gas. Specifically, the vacuum container is separated into a plasma generating space and a film forming space using a partition plate which has a plurality of holes through which the radicals pass. Gas is introduced into the plasma generating space. Radicals are generated from plasmas and these generated radicals are introduced into the film forming space through the holes of the partition plate. Material gas is directly introduced into the film forming space (i.e., directly introduced from the outside of the vacuum container into the film forming space without contacting the material gas with the plasmas and radicals). The radicals and the material gas thus introduced into the film forming space are caused to react with each other in the film forming space, thereby forming a film on a substrate (which is, for example, a glass substrate of 370 mm×470 m) arranged in the film formation space.
The silicon oxide film formation reaction in the film forming space of the RS-CVD system of this type occurs by contacting atomic oxygen (excited active species) supplied from the plasma generating space to the film forming space with silane (SiH4) gas in the film forming space thereby decomposing the silane gas, and repeating the reaction of the decomposed gas with atomic oxygen, oxygen gas or the like. In the attached FIG. 4, a typical view of these reactions is disclosed.
In other words, in the reaction process shown in FIG. 4, the atomic oxygen (excited active species) generated in the plasma generating space acts both as a trigger which triggers a series of formation reactions for a silicon oxide film and as a reactive species which makes reactions for forming the silicon oxide film.
From these facts, it is known that if the amount of the atomic oxygen introduced from the plasma generating space into the film forming space is small, intermediate products produced as a result of insufficient decomposition of the silane (SiH4) gas mix in a film which is being formed, resulting in the degradation of the quality of the film.
It is possible to improve the efficiency of generating atomic oxygen, which plays an important role in this silicon oxide film formation process, by changing film formation conditions as follows.
FIG. 5 shows the dependency of the flow rate of atomic oxygen flowing from the plasma generating space to the film forming space and measured in the film forming space on discharge power (with a discharge frequency of 60 MHz). As is seen from FIG. 5, the atomic oxygen flow rate does not invariably increase even if discharge power increases and, after reaching a maximum at a discharge power of about 35 W, the atomic oxygen flow rate invariably decreases.
In addition, while the amount of the atomic oxygen generated in the plasma generating space increases as the amount of flow of oxygen gas introduced into the plasma generating space increases at first, it is known that the amount of the atomic oxygen reaches a maximum when the oxygen gas has a certain amount of flow. In the RS-CVD system shown in FIGS. 1 and 2, it is found that the atomic oxygen has a degree of dissociation of about 15% of the total introduced oxygen gas.
On the other hand, as a method for improving efficiency for generating atomic oxygen besides the method for optimizing film formation conditions, disclosed in the Japanese Patent Unexamined Application Publication No. 11-279773 (JP-A-11-279773) there is known a method for increasing the amount of atomic oxygen in plasmas by adding so-called noble gas such as helium (He), krypton (Kr) or argon (Ar).
If this method is used, however, the noble gas is added so as to be high in proportion relative to the oxygen gas.
To improve efficiency for generating target atomic oxygen, noble gas, e.g., krypton which is 20 times as large as oxygen gas or argon (Ar) gas which is 25 times as large as oxygen gas in quantity is added. It is assumed that atomic oxygen is generated by adding argon (Ar) and oxygen gas (O2) at a ratio of argon to oxygen of, for example, 25:1. In this case, even if the oxygen gas has a degree of dissociation of 100%, the proportion of the generated atomic oxygen relative to the total amount of flowing argon and oxygen gas is less than 4% at most.
As stated above, there are some known methods for improving efficiency for generating atomic oxygen which play an important role in the silicon oxide film formation process. However, atomic oxygen cannot be obtained sufficiently only by optimizing silicon oxide film process parameters.
Furthermore, according to the method disclosed in the JP-A-279773, it is necessary to introduce an overwhelming quantity of noble gas relative to oxygen gas and create a noble gas atmosphere so as to improve the efficiency for generating atomic oxygen. Thus the proportion of generated oxygen gas relative to the total flow rate is low. In the present industry in which the areas of substrates become increasingly large, e.g., in a system which forms a silicon oxide film on a 1 meter size large area substrate, a large quantity of noble gas several times or several tens of times as large as that of oxygen gas must be introduced to generate atomic oxygen.